1. Field of the Invention
The present invention relates generally to a process for fabrication of rolled aluminium alloy products with high toughness and high fatigue resistance, and products made using such a process. In particular, the instant process comprises refining liquid metal as well as providing sheets or light-gauge plates that may, for example, be used in aircraft fuselage skins and related applications.
2. Description of Related Art
It is generally known that the various properties required during the manufacture of semi-finished products and structural elements for aircraft construction typically cannot all be optimized at the same time independently of each other. When the chemical composition of the alloy or parameters of product production processes are modified, several critical properties may even change in conflicting trends. This is the case particularly for properties included under the term “static mechanical properties” (particularly the ultimate strength Rm and the yield stress Rp0.2) on the one hand, and properties included under the term “damage tolerance” (particularly toughness and resistance to fatigue crack propagation) on the other hand. Furthermore, some working properties such as fatigue resistance, resistance to corrosion, formability and elongation at failure are linked to the static mechanical properties in a complex and frequently unpredictable manner. Therefore, optimization of all the properties of a material for mechanical construction, for example in the aeronautical sector, frequently depends on a compromise between several key parameters.
For example, Al—Si—Mg—Cu type alloys can be used for structural elements of fuselages for wide body civil aircraft. First, these elements generally should have high mechanical strength, and secondly, possess high toughness and high fatigue resistance. Any new possibility of improving one of these groups of properties without degrading the others would be desirable.
Up to now, efforts made have focused on optimizing the chemical composition of alloys, and optimizing sheets or plate transformation conditions; in other words optimizing rolling and heat treatment sequences.
It was well known that reducing iron and silicon impurities in alloys in the 2xxx and 7xxx series increases the toughness (see J. T. Staley “Microstructure and Toughness of High-Strength Aluminium Alloys” published in the book “Properties Related to Fracture Toughness”, ASTM Special Technical Publication 65, 1976, pp 71-103). In some cases, the reduction of Fe and Si also tends to increase fatigue resistance.
There are few studies related to the influence of conditions for refining of liquid metal and casting of as-cast forms (such as billets and ingots) on the toughness of ingots obtained from such as-cast forms.
EP 1 205 567 A (Alcoa Inc.) teaches that the addition of 0.003 to 0.010% of Ti and Boron or Carbon to a wrought alloy will result in cast grain sizes of 200 μm or less.
U.S. 2002/0011289 A1 (Pechiney Rhenalu) teaches that for thick products with only a slightly recrystallized microstructure (in other words in which the fraction of recrystallized grains is less than 35%), a high as-cast grain size could lead to a specific microstructure of the transformed and heat-treated product that has a beneficial effect on toughness. This result is obtained particularly by careful control of the titanium and boron content, these elements being added in the form of TiB2 to refine the metal grain during solidification.
U.S. Pat. No. 5,104,616 (Baeckerud) particularly addresses problems that arise due to hard boride particles in the beverage can and thin aluminium sheet industries and teaches that it may be advantageous to replace a refining agent containing boron with a refining agent containing carbon. However, problems that arise in the aluminium packaging industry such as pin-holes, are incomparable with problems that arise in the aeronautical industry, where product strength and durability are of the utmost importance.
A purpose of the present invention was the provision of a new process for producing highly recrystallized wrought products, preferably rolled products, and particularly sheets or light-gauge plates made of an alloy in the 6xxx series with high mechanical strength that also have excellent toughness and fatigue resistance.